This invention relates to lithographic ink additives.
The printing surface of a lithographic printing plate consists of oleophilic image areas receptive to a greasy ink and lipophobic non-image areas unreceptive to said ink. The non-image areas are a generally hydrophilic surface receptive to water. Accordingly, the common lithographic printing is performed by feeding both an ink and water to the printing surface so that the image areas receive preferentially the coloring ink and the non-image areas preferentially receive the water, and transferring the ink in image areas to a substrate such as, for example, paper. As the lithographic printing plates using a dampening solution, there are known a presensitized plate utilizing diazonium compounds, a printing plate of the electrophotographic type employing zinc oxide or an organic photoconductor, and a printing plate of the silver salt photographic type using a silver halide emulsion as photosensitive component.
For use in various lithographic printing plates, there are also known a large variety of printing inks. Although conventional lithographic inks are capable of producing good prints in the cases where a specific printing plate or a specific dampening solution is used, yet they reveal various disadvantages under other printing conditions such as variation in the type of printing plate or dampening solution and even become unsuitable for practical use. The requisite properties for the lithographic inks include sufficient adherence to the image areas, perfect non-adherence to the non-image areas, and, in addition to these obviously fundamental properties, suitable levels of those properties which relate to flow, interfacial behavior, and drying. Especially in lithographic printing, the greasy ink is required to maintain an adequate balance between the ink-water interfacial tension and the surface tensions of both phases, because otherwise there will occur gradual enlargement or, conversely, disappearance of the image areas, emulsification of the ink, and a phenomenon of scumming during the printing of thousands of tens of thousands of copies with alternate feeding of a greasy ink and water, which is repeated for each copy. Therefore, printing inks of excellent interfacial behavior are required in lithography. Conventional lithographic inks, which are hardly said to have satisfactory interfacial behavior, tend to cause the above-mentioned disadvantages and are insufficient in the adaptability to variation in the plate material and dampening solution. Particularly, most color inks, excepting a black ink, used in color printing contain comparatively hydrophilic pigments, as contrasted with carbon black, and are liable to emulsification during printing, resulting in severe scumming which interferes with satisfactory tone reproduction and makes the prints useless.